A great many analytical aids are known for rapid and quantitative analytical determination of constituents of liquid samples. The analytical aids can be in the form of individual test elements such as test strips or strip-shaped test materials, as well as in integrated systems in which the testing element is a part of a sampling device. For reasons of simplified and more economical production and component stability, such analytical aids typically are made of plastic or metal and generally have a comparatively hydrophobic surface, which is a disadvantage for rapid and uniform wetting of the analytical aid with liquid samples.
Int'l Patent Application Publication No. WO 2007/045412 describes sampling devices for collecting body fluids that have a needle element and that are equipped with a testing element, with which at least one analyte in the body fluid can be detected qualitatively or quantitatively. In this case, the body fluid sample is transported along the needle element to the testing element. Because the site of insertion of the needle element into the skin is disposed at some distance from the testing element, the body fluid sample must travel a few millimeters (mm) along the capillary to reach the testing element. The needle elements are made of steel and therefore have a relatively hydrophobic steel surface and thus are not very suitable for transporting the body fluid sample.
For the purpose of increasing the flow rate of the body fluid sample along the needle element, EP Patent Application Publication No. 2025287 describes modifying the surface of the needle element with a hydrophilic coating. Non-ionic surfactants, such as polysorbate, are mentioned for use as the hydrophilic coatings.
Similarly, EP Patent Application Publication No. 2014727 describes hydrophilizing metal-oxide coatings such as, for example, AlOOH, TiOX, SiO2 or the like. The metal-oxide coatings can be applied and fixed in the form of discrete particles in suspensions. The use of organic polymer compounds such as, for example, PVP-PEG, and of water-soluble organic poly-acids and/or salts thereof such as, for example, PAA or heparin salts, as the hydrophilic coating, also is known.
For forming a hydrophilic surface of the needle element, hydrophilization by physicochemical means also is possible. For example, etching of a steel surface or a plasma or corona treatment can be used to create active metal surfaces. Unfortunately, these effects are temporary, and therefore only are auxiliary measures for preparing the needle element surface for subsequent coating.
Likewise, a capillary-active surface structure can be provided to improve the transport of the body fluid sample along the needle element, where a hydrophilic coating of the capillary surface structure can be especially advantageous for speed of transport of the body fluid along the needle element.
In this manner, EP Patent Application Publication No. 1887355 describes a needle element designed as a microfluidic system for capillary transport of a body fluid sample and a method of applying a hydrophilic surface coating on the needle element. Polyacrylic acid, polyacrylate, dextran sulphate and/or chondroitin sulphate are mentioned for use as the hydrophilic surface coating materials. To ensure rapid and reliable transport of the body fluid sample along the needle element, a surface structure is further described that can be formed as a microchannel on the needle element with the hydrophilic surface coating.
However, the known coating materials demonstrate inadequate stability, in particular, they have inadequate long-term stability. One reason for the inadequate stability includes insufficient bonding of the coating to the surface of the substrate, which may result in detachment of the coating during the coating process, during storage or during use of the substrate such as, for example, when a needle element coated in this way is inserted into the skin. Another reason is that the hydrophilic coating may not even be resistant to longer storage at room temperature or to temperature fluctuations. Moreover, some compounds display low stability when exposed to sterilization conditions and/or low resistance to gas evolution from packaging.
In particular, instability with respect to packaging materials often means that in the case of substrates packaged in plastic materials, the hydrophilicity of the coated surfaces decreases markedly during storage and sufficient hydrophilicity of the substrates after storage is no longer guaranteed. This loss of hydrophilicity can be explained by adsorption of volatile, nonpolar constituents of the packaging materials.
To prevent such loss of hydrophilicity, Int'l Patent Application Publication No. WO 2008/015227 describes using specific packaging that contains a loose covering for the hydrophilic coating and/or the addition of adsorbents in the packaging to prevent absorption of volatile constituents by the hydrophilic coating. Unfortunately, the packaging is complicated, expensive and, in particular, is disadvantageous for measuring equipment with automatic operation, which also must provide reliable unpacking of the substrate from the packaging.
US Patent Application Publication No. 2007/0179373 describes apparatuses and methods for collecting a body fluid sample. It discloses inter alia an integrated collection device having an analyte detector and a gradient means to drive the transport of the body fluid from a point of contact to a point of detection. The integrated collection device has a layered structure that includes multiple layers and features a first layer and a second layer that forms a surface layer. A hydrophilic surface is generated by powder coating the surface layer with fumed silica nanoparticles.
For the foregoing reasons, there is a need for analytical aids with hydrophilic coatings that have sufficient stability, in particular, sufficient stability even in the presence of volatile, nonpolar constituents of packaging materials.